A standard propeller such as usable as the drive screw of a watercraft comprises a central hub centered on and normally rotated about a hub axis, and a plurality of identical blades extending radially at equiangular spacing from the hub. Such a propeller is invariably made by casting from one piece of metal, either stainless steel or a cuproaluminum alloy, as it must be very strong and must also be capable of being finished to very close tolerances.
The rough casting from which the propeller is eventually made is cast in a mold that is made from a model supplied to the foundry either in wood or in metal. The wood model normally has a single blade but the metal one is complete.
The wood-model route is most popular because the model costs the least to make. The foundry has to make up individual molds for each blade. In this there is a possibility of modifying the pitch by inclining the axis of the propeller hub relative to the original setting, but this possibility is limited and anyhow the new value of the pitch is really only obtained on a single radius normally equal to seven-tenths of the radius of the propeller.
Once the rough casting for a given propeller is completed it is subject to a complex and expensive machining operation that must be carried out by hand by skilled workers. The propeller must be ground and eventually statically and even dynamically balanced. As a result a propeller is expensive to manufacture. In addition propellers must also normally be produced in a wide range of sizes.
It is fairly common for a propeller to be damaged, in particular in inland waterways, so that the propeller must either be repaired or replaced. Repair is extremely exacting work, and replacement is also expensive because of how much a propeller costs to make in a group of different sizes and then to stock in this range of sizes.
It is also common that a change in engine or operating conditions requires that a different type of propeller be used. In this situation a perfectly good propeller must be replaced with the needed type, or inferior operating performance with the existing but now mismatched propeller must be tolerated.